JP2006286313A - Non-water electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a non-water electrolyte secondary battery suppressing expansion due to deterioration of negative electrode active substance containing silicon and showing a superior characteristic of charging/discharging cycle. <P>SOLUTION: The non-water electrolyte secondary battery is provided with the negative electrode containing the silicon as the negative electrode active substance, a positive electrode containing a positive electrode active substance, non-water electrolyte and a separator. An additive suppressing the oxidation of the silicon in operation of the cell is contained in the separator or on the surface of the separator. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  In recent years, portable electric devices have been remarkably reduced in size and weight, and power consumption has increased with the increase in functionality. For this reason, in the lithium secondary battery used as a power source, there is a strong demand for light weight and high capacity.

  In response to such demands, in recent years, silicon has been proposed as an electrode material that is excellent in unit mass and charge / discharge capacity per unit volume as compared with a carbon negative electrode.

  In a conventional lithium secondary battery using a silicon thin film as a negative electrode active material, an electrode of an active material thin film having a columnar structure separated into a columnar shape by forming a cut in the thickness direction has been proposed. With such a columnar structure, stress due to expansion and contraction of the active material can be relieved, the active material can be prevented from being detached from the current collector, and charge / discharge cycle characteristics can be improved. it can.

  However, in a negative electrode using such a silicon thin film, it is known that the active material is altered and made porous by repeating charge and discharge cycles. As will be described later, the present inventors have found that such alteration of the active material is due to oxidation of silicon during battery operation. The present invention is based on such knowledge of the present inventors.

  In patent document 1, in order to prevent the oxidation of the silicon | silicone at the time of manufacturing an electrode, using a pH adjuster is proposed. However, Patent Document 1 does not disclose or suggest any silicon oxidation in the charge / discharge cycle.

Patent Document 2 describes that a saturated dicarboxylic acid is added to the inside of the negative electrode to improve charge / discharge cycle characteristics. Patent Document 3 describes that an organic acid is added inside the negative electrode to improve cycle characteristics. However, these prior arts do not disclose any improvement in charge / discharge cycle characteristics by suppressing prevention of silicon oxidation.
JP 2004-349079 A Japanese Patent Laid-Open No. 2004-6188 JP 2004-335379 A

  An object of the present invention is a nonaqueous electrolyte secondary battery containing silicon as a negative electrode active material, wherein the expansion of the negative electrode active material containing silicon is suppressed, and exhibits excellent charge / discharge cycle characteristics. Is to provide.

  The present invention is a non-aqueous electrolyte secondary battery comprising a negative electrode containing silicon as a negative electrode active material, a positive electrode containing a positive electrode active material, a non-aqueous electrolyte, and a separator. It is characterized by containing an additive that suppresses oxidation of silicon during battery operation.

  In the present invention, since an additive for suppressing silicon oxidation during battery operation is included in the separator or on the surface of the separator, expansion due to deterioration of the negative electrode active material containing silicon is suppressed, and charge / discharge cycle characteristics Can be improved.

In the present invention, the additive for suppressing silicon oxidation during battery operation includes an acid, a weak alkali, an acid anhydride, or a lithium salt of an acid. As will be described later, the present inventors have found that the oxidation of silicon during battery operation is promoted by a reaction similar to the reaction by OH ⁻ . In order to suppress such oxidation of silicon, it is only necessary to suppress the oxidation reaction of silicon by alkali by making the atmosphere in which silicon exists weakly alkaline or acidic, and the additive used in the present invention is silicon. It is a substance that can make the atmosphere present weakly alkaline or acidic.

  Specific examples of additives in the present invention include acid anhydrides such as succinic anhydride and acetic anhydride, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, Carboxylic acids such as phthalic acid and fumaric acid, and dicarboxylic acids and salts or esters thereof, hydroxy acids, carbonates such as lithium carbonate, nitrates such as lithium nitrate, sulfonates such as lithium sulfonate, acrylic acid and derivatives thereof, methacrylic acid And derivatives thereof, compounds that form free fluorine in the electrolyte, and salts formed by the reaction of strong acids and weak alkalis.

  Further, in the present invention, since the additive is contained in the separator, even if the negative electrode or the positive electrode contains a compound that easily reacts with the additive, the additive does not cause a problem in the battery. Can be included.

In the present invention, the content of the additive with respect to the separator is preferably in the range of 1 × 10 ⁻⁶ to 1 × 10 ⁻³ g per 1 cm ² of the separator. When the amount of the additive is more than 1 × 10 ⁻³ g per 1 cm ² of the separator, the initial capacity of the battery is lowered, and as a result, the energy density may be lowered. This seems to be because the diffusion of lithium ions in the separator is hindered by the increase in the amount of the additive. On the other hand, if the amount of the additive is less than 1 × 10 ⁻⁶ g, the effect of the present invention for improving the charge / discharge cycle characteristics may not be sufficiently obtained.

  The separator in the present invention is not particularly limited as long as it is a separator that can be used for a lithium secondary battery. For example, a polyolefin microporous film represented by polyethylene or polypropylene, a composite film thereof, or polyamide is used. For example, a non-woven fabric made of fine fibers such as glass fibers and glass fibers can be suitably used.

  Further, in the present invention, as a method of adding an additive to the separator, for example, in order to contain the additive on the separator surface, a solution in which a solid additive pulverized by pulverization is suspended is sprayed on the separator surface. After that, the solvent is removed by performing a reduced pressure treatment, and the additive is uniformly dispersed on the surface, whereby the additive can be uniformly contained on the separator surface.

  Furthermore, in order to retain the additive inside the separator, it is possible to retain the additive inside the separator by removing the solvent by drying under reduced pressure after impregnating the separator in a solution in which the additive is dissolved. Become.

  In accordance with the present invention, by adding the additive in the separator surface or the surface of the separator, the additive can be stably supplied to the electrolyte solution near the surface of silicon as the negative electrode active material, and the component that oxidizes silicon It can be consumed continuously and the oxidation of silicon can be suppressed continuously.

  The negative electrode in the present invention is preferably an electrode formed by depositing a thin film made of silicon or a silicon alloy on a current collector. Such a thin film can be formed by sputtering, CVD, vapor deposition, thermal spraying, or the like. Since such a thin film expands in the thickness direction due to a charging reaction, it is considered that a portion of the active material covered with the protective film is peeled off and the surface of the new silicon active material is likely to come into contact with the electrolytic solution. As a result, reaction with an electrolytic solution or the like is facilitated, and silicon is oxidized. By suppressing the oxidation of silicon under such circumstances, the expansion of the active material due to the charge / discharge cycle can be suppressed, and an increase in the thickness of the active material layer can be suppressed.

  The thin film made of silicon or a silicon alloy preferably has a columnar structure separated into columns by cuts formed in the thickness direction. By having such a columnar structure, the stress due to the expansion and contraction of the active material at the time of insertion and desorption of lithium into the active material is relieved, and the active material layer is prevented from peeling and dropping from the current collector. is there. By suppressing the oxidation of silicon in the active material that repeats such expansion and contraction, expansion of silicon and cycle deterioration can be suppressed.

  In order to form a cut in the thickness direction as described above, it is preferable that irregularities are formed on the surface of the thin film. In order to form irregularities on the surface of the active material thin film, it is preferable to use a roughened copper foil or the like as the negative electrode current collector. Examples of such copper foil include electrolytic copper foil. The electrolytic copper foil is obtained, for example, by immersing a metal drum in an electrolyte solution in which ions are dissolved, and by flowing an electric current while rotating the drum to deposit metal on the surface of the drum and peeling it. Foil. A metal may be deposited on one side or both sides of the electrolytic copper foil by an electrolytic method to roughen the surface. Instead of these, a metal may be deposited on the surface of the rolled foil by an electrolytic method to roughen the surface.

  In the present invention, the negative electrode active material may be an alloy of silicon and another metal. Examples of other metals include cobalt, zirconium, zinc, iron, and the like, and an alloy of silicon and cobalt is particularly preferably used. By containing cobalt, the charge / discharge cycle can be further improved. In an alloy of silicon and another metal, it is preferable that 50 atomic% or more of silicon is contained.

  In the present invention, the additive is preferably in a solid state in the non-aqueous electrolyte. That is, the additive in the present invention is preferably a substance that is difficult to dissolve in the non-aqueous electrolyte. Since the additive is consumed by reacting with a component that increases the amount of silicon oxide, it is preferable that the additive is constantly present in the electrolytic solution. If the additive is easily dissolved in the electrolytic solution, elution of the additive into the electrolytic solution increases, and the additive is excessively present in the electrolytic solution. In such a case, the additive is consumed due to a reaction with lithium or the like in silicon, and the additive may not be present in the subsequent charge / discharge cycle.

  By making the additive which exists in a solid state in nonaqueous electrolyte solution, an additive will melt | dissolve gradually in electrolyte solution, and only what melt | dissolved will contribute to reaction. For this reason, the quantity consumed by a side reaction can be suppressed to the minimum. For this reason, it is thought that an additive can be used efficiently and the effect according to the addition amount can be acquired.

  In the present invention, it is preferable that a film forming agent for forming a film on the surface of the negative electrode is contained in the non-aqueous electrolyte. In such a film-forming agent, the film formed by reductive decomposition of the film-forming agent improves the uniformity of lithium desorption / insertion reaction on the negative electrode surface, and suppresses the progress of local deterioration and side reactions. It is considered effective. However, this type of film cannot sufficiently suppress the oxidation of the silicon surface, and if the film forming agent is not present in the battery, the effect as a film cannot be continued. When silicon is oxidized, the negative electrode active material expands and the surface of the active material increases, so the amount of film destruction increases and the consumption of the film forming agent increases. The number of charge / discharge cycles obtained is shortened. In order to continuously obtain the effect of adding the film forming agent, it is necessary to simultaneously suppress the swelling of the negative electrode active material. Therefore, by using the additive and the film forming agent in the present invention in combination, the advantages of both can be efficiently exhibited, a synergistic effect can be obtained, and excellent cycle characteristics can be obtained.

  Specific examples of the film forming agent as described above include vinylene carbonate (VC) and vinyl ethylene carbonate (VEC). Further, ethylene carbonate (EC) also functions as a film forming agent when used in a high temperature environment.

  The amount of the film-forming agent added is preferably 0.01 to 10% by weight, more preferably 0.1 to 10% by weight with respect to the non-aqueous electrolyte.

  In the present invention, the additive may be contained in the holding body. Specifically, both inorganic materials and organic materials can be suitably used as the holding body as long as it is a substance that has pores inside and is stable in the battery, such as inorganic solid fillers such as alumina, titania, and silica. .

  The same applies to the use of a capsule having an outer shell that gradually dissolves in the electrolyte, and particularly when a liquid additive is used, the polymer polymer material that can retain the additive by swelling. Thus, it can be suitably used. By holding the additive in the holding body, it becomes easy to control the dissolution of the additive in the electrolytic solution. When the additive is solid, a holding body having a void inside can be used to hold the additive in the void. Further, when the additive is liquid, the holding body can be impregnated with the additive.

  The holding body is preferably solid fine particles. By using such a holding body, the strength of the separator can be improved and a thin film can be formed. Therefore, the additive can be held in the separator without reducing the energy density.

  Examples of the non-aqueous solvent used in the non-aqueous electrolyte in the present invention include cyclic carbonates, chain carbonates, lactone compounds (cyclic carboxylic acid esters), chain carboxylic acid esters, cyclic ethers, chain ethers, And sulfur-containing organic solvents. Among these, preferably, a cyclic carbonate having 3 to 9 carbon atoms, a chain carbonate, a lactone compound (cyclic carboxylic acid ester), a chain carboxylic acid ester, a cyclic ether, and a chain ether are mentioned. It is preferable to include one or both of a cyclic carbonate having 3 to 9 carbon atoms and a chain carbonate.

  As the solute in the nonaqueous electrolytic solution in the present invention, a lithium salt compound generally used in lithium secondary batteries can be used.

  The positive electrode active material used in the present invention may be any positive electrode active material that can be used for a lithium secondary battery. For example, lithium cobalt oxide, lithium manganate, lithium nickelate, and lithium transition metals containing these oxides Examples include complex oxides. These oxides may be used alone or in combination of two or more.

<Oxidation of silicon during battery operation>
Hereinafter, the oxidation of silicon during battery operation will be described.

  FIG. 1 is a secondary electron image showing a fracture surface of an electrode of a silicon thin film before charging and discharging. The silicon thin film electrode is an electrode formed by depositing a silicon thin film on the electrolytic copper foil as a current collector by a sputtering method.

  FIG. 2 is a secondary electron image showing a state after charging in the first cycle, and FIG. 3 is a secondary electron image showing a state after discharging in the first cycle.

  As is clear from FIGS. 2 and 3, when lithium is inserted into the silicon thin film and charged, silicon expands more than twice in the thickness direction. Next, when discharge is performed, the silicon thin film cracks in the thickness direction to form a columnar structure, and the thickness is increased to 1.5 times or more of that before charge / discharge. By having such a columnar structure, stress due to expansion and contraction of the active material can be relieved, and the active material can be prevented from dropping from the current collector.

  When a charge / discharge cycle is repeated using such a negative electrode of a silicon thin film, the discharge capacity retention rate decreases with the charge / discharge cycle, as shown in FIG.

  5 to 7 are scanning ion microscope images showing cross sections of the silicon thin film electrodes before and after charge / discharge, before discharge capacity deterioration (after 10 cycles), and after discharge capacity deterioration (after 100 cycles). These are microscopic images when a tungsten protective film is deposited on the surface of the active material layer, then the active material layer is cut by a focused ion beam device, and a cross section of the active material column is observed with a scanning ion microscope. 5-7, the dashed-dotted arrow has shown the tungsten protective film.

  5 to 7, a portion indicated by a solid line arrow is an altered portion, which is a portion having a brighter contrast than the inside of the columnar active material, and is a region where the active material is altered. A portion indicated by a dotted arrow is an unaltered portion, an internal region of a columnar portion having a low contrast, and a region where the active material is not deteriorated.

  As apparent from FIGS. 5 to 7, the altered portion of the active material is small before charging and discharging and before deterioration of the discharge capacity, but is increased after deterioration of the discharge capacity. In the region of the altered portion, the void density is reduced (expanded) due to the formation of voids in the active material, and thus the thickness of the active material layer is increased. Note that no pulverization was observed, and no active material was removed from the current collector.

  After cleaning the negative electrode with dimethyl carbonate and vacuum drying, the surface of the negative electrode active material and internal silicon and oxygen were analyzed in the depth direction by X-ray photoelectron spectroscopy (XPS) combined with sputtering with an argon ion beam It was.

8 to 10 are diagrams showing the ratio of the silicon concentration and the oxygen concentration with respect to the sputtering time in terms of 100 fractions relative to the number of atoms. FIG. 8 shows before charge / discharge, FIG. 9 shows before discharge capacity deterioration, and FIG. 10 shows after discharge capacity deterioration. The sputter rate is 10 nm / min in terms of SiO ₂ .

  As apparent from FIG. 8 to FIG. 10, before charging / discharging and before the discharge capacity deterioration, a large amount of oxygen is present only on the outermost surface, but after the discharge capacity deterioration, the sputtering time is 80 minutes (the outermost surface). It can be seen that oxygen is present at 20 atomic% or more up to a deeper part (800 nm) or more, and that oxidation of silicon proceeds by charge / discharge cycles.

  Therefore, the altered portion indicated by the solid arrow shown in FIGS. 5 to 7 is a region where oxygen is present at a high concentration, and is considered to be silicon oxide. Moreover, it is thought that the unaltered part shown with the dotted-line arrow is maintaining the state which silicon is not oxidized.

In X-ray photoelectron spectroscopy (XPS) analysis, the valence of silicon is relative to the binding energy position of the XSP Si-2p spectrum.
Zero-valent silicon: approx. 99 eV
Divalent silicon: approx. 101 eV
Tetravalent silicon: about 103eV
The relationship is known.

  FIG. 11 shows the XPS Si after the outermost surface of the negative electrode silicon thin film after discharge capacity deterioration, after 1 minute sputtering, after 10 minutes sputtering, after 20 minutes sputtering, after 40 minutes sputtering, and after 80 minutes sputtering. -2p spectrum. As is clear from FIG. 11, it can be seen that the surface of the silicon thin film has a lot of divalent silicon and the inside has a lot of zero-valent silicon.

  12 to 14 show the XPS Si-2p spectrum peak area in the negative electrode silicon thin film before charge / discharge (FIG. 12), before discharge capacity deterioration (FIG. 13), and after discharge capacity deterioration (FIG. 14). It is a figure which shows the XPS profile which isolate | separated into 0 valence silicon and silicon oxide (total of 2 valence silicon and 4 valence silicon), and calculated | required the ratio of 0 valence silicon and silicon oxide as 100 fraction with respect to the number of atoms.

  As is apparent from FIGS. 12 to 14, silicon oxide exists only near the outermost surface before charge / discharge and before charge / discharge deterioration, but after discharge capacity deterioration, a deep region with a sputtering time of 80 minutes is present. Thus, it can be seen that silicon oxide is formed at a ratio of 20 atomic% or more.

  As described above, it has been confirmed that silicon is oxidized by the charge / discharge cycle, and the thickness of the active material layer is increased due to the expansion of the active material. One reason for the deterioration of the discharge capacity is considered to be that the electrical conductivity of the active material surface is lowered due to oxidation and expansion of the surface of the active material, and the resistance value in the insertion and extraction of lithium is increased. The following reaction can be considered as a reaction for oxidizing such silicon.

Si + 2Li ⁺ + 2OH ⁻ → Si (OLi) ₂ + H ₂ ↑ (1)
2Si + 6OH ⁻ → 3SiO ₂ ²⁻ + 3H ₂ ↑ (2)

The above reaction formula is a reaction formula generally known in an aqueous solution system, and in the aqueous solution system, OH ⁻ causes an oxidation reaction of silicon.

However, in the organic solvent used as the non-aqueous electrolyte, it hardly exists in the form of OH ⁻ , such as LiOH, ROLi, Li ₂ O, ROCO ₂ Li, and RCO ₂ Li produced on the surface of the negative electrode active material. According to OH ⁻ in the above formulas (1) and (2), a substance that shows alkalinity in a Li-containing compound causes a reaction similar to the reaction according to OH ⁻ in the above formulas (1) and (2) to increase silicon oxide. It is thought that.

In a state where lithium is present, OH by reaction with lithium moisture also represented by the following reaction scheme present in the system ^- occurs, which is likely to cause an oxidation reaction on the silicon.

2Li + 2H ₂ O → 2Li—OH + H ₂ ↑ (3)

  The component that increases silicon oxide generated in the battery is considered to be a substance that causes the following reactions (i) and (ii) by reaction with silicon.

(i) Reaction in which the oxidation number of silicon increases from 0 valence to 2 or 4 valence, that is, oxidation reaction of silicon (ii) Silicon as an active material because it is more than a reaction that produces a compound having a bond between silicon and oxygen It has been found that the deterioration of is caused by the oxidation reaction of silicon. In the present invention, based on such knowledge of the present inventors, an additive that suppresses silicon oxidation is included in the separator or on the surface of the separator, thereby suppressing silicon deterioration and improving charge / discharge cycle characteristics. I am letting.

  According to the present invention, by including an additive that suppresses oxidation of silicon during battery operation in the separator or on the surface of the separator, expansion (expansion) is suppressed due to deterioration of the active material containing silicon, which is excellent. Charge / discharge cycle characteristics can be obtained.

  Hereinafter, the present invention will be described with reference to specific examples, but the present invention is not limited to the following examples.

Example 1
[Production of positive electrode]
Lithium cobaltate as a positive electrode active material, ketjen black as a conductive additive, and fluororesin as a binder are mixed in a mass ratio of 90: 5: 5, and this is mixed with N-methyl- It melt | dissolved in 2-pyrrolidone (NMP) and produced the positive mix paste.

  The prepared positive electrode mixture paste was uniformly applied to both surfaces of an aluminum foil having a thickness of 20 μm by a doctor blade method. Next, the NMP was removed by vacuum heat treatment at a temperature of 100 to 150 ° C. in a heated drier, and then rolled with a roll press so that the thickness was 0.16 mm to produce a positive electrode.

(Production of negative electrode)
As the current collector, an electrolytic copper foil having a thickness of 18 μm and a surface roughness Ra of 0.188 μm was used. On this electrolytic copper foil, sputtering gas (Ar) flow rate: 100 sccm, substrate temperature: room temperature (no heating), reaction pressure: 0.133 Pa (1.0 × 10 ⁻³ Torr), high frequency power: 200 W A silicon thin film having a thickness of 5 μm was formed by RF sputtering. As a result of measuring the obtained silicon thin film using Raman spectroscopy, a peak in the vicinity of a wavelength of 480 cm ⁻¹ was detected, but a peak in the vicinity of 520 cm ⁻¹ was not detected. From this, it was confirmed that it was an amorphous silicon thin film.

  As described above, a silicon thin film was formed on both sides of the current collector and used as a negative electrode.

[Preparation of separator]
What adsorb | sucked ester on the powder silica surface and polyethylene powder were mixed, and it formed into a film by the melt extrusion method, and obtained the sheet | seat of thickness 200 micrometers. Next, the obtained sheet is immersed in a 20% by weight aqueous solution of caustic soda and an organic solvent, silica powder and ester are extracted and removed, further washed with water and dried, and then in the MD (Machine direction) and TD (Transmachine direction) directions. By stretching, a separator made of a polyethylene microporous film having a thickness of 20 μm and having a micropore inside was obtained.

Prepare a solution in which succinic anhydride is dissolved in diethyl carbonate to a concentration of 1 mol / liter, impregnate the above separator in this solution, fully impregnate, then dry under reduced pressure to obtain diethyl carbonate. The succinic anhydride was uniformly dispersed in the microporous portion of the separator. The weight of the separator was measured before and after the succinic anhydride content treatment to determine the content of succinic anhydride in the separator. The content of succinic anhydride was 1 × 10 ⁻⁵ g / cm ² .

[Preparation of non-aqueous electrolyte]
LiPF ₆ as an electrolyte salt was dissolved in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed so as to have a volume ratio of 3: 7 so as to be 1 mol / liter, thereby preparing an electrolytic solution.

[Production of secondary battery]
The positive electrode and the negative electrode were cut into a predetermined size, and a current collecting tab was attached to each current collector. The separator was sandwiched between a positive electrode and a negative electrode, and this was wound up, and the outermost periphery was fastened with a tape to form a spiral electrode body, which was then flattened into a plate-like body. This is inserted into an exterior body made of a laminate material made by laminating PET and aluminum, and then injected with the electrolyte, and then sealed with the tab protruding outward from the end, and the lithium secondary A battery was produced.

(Comparative Example 1)
A lithium secondary battery was produced in the same manner as in Example 1 except that a separator not containing succinic anhydride as an additive was used.

[Evaluation of cycle characteristics]
The batteries of Example 1 and Comparative Example 1 were evaluated for cycle characteristics. Charge / discharge up to the 50th cycle under the following charge / discharge cycle conditions, measure the discharge capacity after the initial and 50th cycle, and calculate the discharge capacity retention ratio (initial discharge capacity / discharge capacity after 50 cycles × 100), The results are shown in Table 2.

Charging condition: 250 mA-4.2 V 12 mA termination constant current-constant voltage charging Discharging condition: 250 mA-2.75 V termination constant current discharging

[Measurement of thickness of active material layer]
Under the same charge / discharge cycle conditions as described above, charge / discharge was performed up to 80 cycles, and the change in the thickness of the active material layer after 80 cycles was measured. The change in the thickness of the active material layer was determined by observing the negative electrode taken out from the battery before and after the charge / discharge test with a scanning electron microscope (SEM), measuring the thickness of the active material layer, and measuring the thickness of the active material layer. The change of was calculated.

  15 is an SEM photograph showing the negative electrode of Example 1 after 80 cycles, FIG. 17 is an SEM photograph showing the negative electrode of Comparative Example 1 before the charge / discharge test, and FIG. 18 is a comparison after 80 cycles. 2 is a SEM photograph showing the negative electrode of Example 1. The thickness of the active material layer was determined based on the dotted line (position indicated by the arrow) shown in each figure. The measurement results are shown in Table 2.

[Measurement of oxygen / silicon ratio on the negative electrode surface]
The negative electrode after 80 cycles was taken out, washed with dimethyl carbonate, and then subjected to elemental analysis using an electron beam microprobe on the surface of the negative electrode to measure the oxygen / silicon ratio on the negative electrode surface. The measurement results are shown in Table 2.

  As is apparent from the results shown in Table 2, the inclusion of succinic anhydride as an additive for suppressing silicon oxidation according to the present invention in the separator increases the discharge capacity retention rate, improves the cycle characteristics, and improves the active characteristics. It can be seen that an increase in the thickness of the material layer is also suppressed.

(Example 2)
In Example 1, a lithium secondary battery was produced in the same manner as in Example 1 except that a non-aqueous electrolyte to which vinylene carbonate (VC) was added to 2 wt% was used as the non-aqueous electrolyte.

(Comparative Example 2)
In Example 2, a lithium secondary battery was produced in the same manner as in Example 2 except that a separator not containing succinic anhydride was used.

[Measurement of charge / discharge cycle characteristics and thickness change of negative electrode active material layer]
In the same manner as in Example 1, the charge / discharge cycle characteristics were evaluated, and the discharge capacity retention rate was shown in Table 4. Further, the thickness change of the negative electrode active material layer was measured in the same manner as in Example 1 and shown in Table 4. FIG. 16 is an SEM photograph showing the negative electrode of Example 2 after 80 cycles, and FIG. 19 is an SEM photograph showing the negative electrode of Comparative Example 2 after 80 cycles. Table 4 also shows the results of Example 1 and Comparative Example 1.

  As is apparent from the results shown in Table 4, by adding vinylene carbonate to the non-aqueous electrolyte and adding succinic anhydride to the separator, the effects of improving charge / discharge cycle characteristics and suppressing electrode swelling are further remarkable. It can be seen that

(Example 3)
In Example 1, instead of using a separator containing succinic anhydride, a separator not containing succinic anhydride was used, and succinic anhydride was contained on the surface of this separator at a concentration of 1 × 10 ⁻⁵ g / cm ² . The amount was dispersed and contained. As a method for inclusion on the separator surface, a solution in which succinic anhydride pulverized by pulverization is suspended in DEC is sprayed on the separator surface, and then the DEC is removed by performing a reduced pressure treatment and added uniformly to the surface. By dispersing the agent, the additive was uniformly added to the separator surface. A lithium secondary battery was produced using a non-aqueous electrolyte containing VC in the same manner as in Example 2 except that such a separator was used.

Example 4
A lithium secondary battery was produced in the same manner as in Example 3 except that in the production of the positive electrode of Example 3, a silicon-cobalt alloy thin film was formed instead of the silicon thin film. The cobalt content in the lithium-cobalt alloy thin film is 20% by weight.

[Evaluation of charge / discharge cycle characteristics]
The lithium secondary batteries of Examples 3 and 4 were evaluated for charge / discharge cycle characteristics in the same manner as described above, and the discharge capacity retention ratios are shown in Table 6.

  As is apparent from the results shown in Table 6, it is understood that the cycle characteristics are further improved by using a silicon-cobalt alloy in which cobalt is contained in silicon as an active material. In addition, it can be seen that the effect of the present invention is also exhibited when succinic anhydride is included in the separator surface.

The secondary electron image which shows the fracture surface of the electrode of the silicon thin film before charging / discharging. The secondary electron image which shows the fracture surface of the electrode of the silicon thin film of the state after the 1st cycle charge in a prior art example. The secondary electron image which shows the fracture surface of the electrode of the silicon thin film of the state after the 1st cycle discharge in a prior art example. The figure which shows the charging / discharging cycle in a prior art example. The figure which shows the scanning ion microscope image of the cross section of the electrode of the lithium thin film before charge. The figure which shows the scanning ion microscope image of the cross section of the electrode of the silicon thin film before the discharge capacity deterioration in a prior art example. The figure which shows the scanning ion microscope image of the cross section of the electrode of the silicon thin film after discharge capacity deterioration in a prior art example. The figure which shows the XPS profile of the silicon and oxygen of the surface of the silicon thin film before charging / discharging. The figure which shows the XPS profile of the silicon | silicone and oxygen of the surface of the negative electrode of the silicon thin film before the discharge capacity deterioration in a prior art example. The figure which shows the XPS profile of the silicon and oxygen of the surface of the negative electrode of the silicon thin film after the discharge capacity deterioration in a prior art example. The figure which shows the XPS Si-2p spectrum of the surface of the negative electrode of the silicon thin film after the discharge capacity deterioration of a prior art example. The figure which shows the XPS profile of zerovalent silicon and the silicon oxide of the surface of the negative electrode of the silicon thin film before charging / discharging. The figure which shows the XPS profile of zerovalent silicon and the silicon oxide of the surface of the negative electrode of the silicon thin film before the discharge capacity deterioration of a prior art example. The figure which shows the XPS profile of the zerovalent silicon and the silicon oxide of the surface of the negative electrode of the silicon thin film after deterioration of the discharge capacity of a prior art example. The scanning electron micrograph which shows the state of the negative electrode after 80 cycles of Example 1 according to this invention. The scanning electron micrograph which shows the state of the negative electrode after 80 cycles of Example 2 according to this invention. The scanning electron micrograph which shows the state of the negative electrode in the charge / discharge cycle test initial stage of the comparative example 1. The scanning electron micrograph which shows the state of the negative electrode after 80 cycles of the comparative example 1. FIG. The scanning electron micrograph which shows the state of the negative electrode after 80 cycles of the comparative example 2.

Claims (10)

In a non-aqueous electrolyte secondary battery comprising a negative electrode containing silicon as a negative electrode active material, a positive electrode containing a positive electrode active material, a non-aqueous electrolyte, and a separator,
A non-aqueous electrolyte secondary battery characterized in that an additive that suppresses oxidation of the silicon during battery operation is contained in the separator or on the surface of the separator.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the additive is an acid, a weak alkali, an acid anhydride, or a lithium salt of an acid.   The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the negative electrode is an electrode formed by depositing a thin film made of silicon or a silicon alloy on a current collector.   The non-aqueous electrolyte secondary battery according to claim 3, wherein the thin film has a columnar structure separated into columns by a cut formed in a thickness direction.   The nonaqueous electrolyte secondary according to any one of claims 1 to 4, wherein the nonaqueous electrolyte contains a film forming agent for forming a film on the surface of the negative electrode. battery.   The non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the negative electrode active material is made of an alloy of silicon and another metal.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the additive is in a solid state in the non-aqueous electrolyte.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the additive is succinic anhydride.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the additive is contained in a holding body. The non-aqueous electrolyte secondary battery according to claim 9, wherein the holding body uses a material made of solid fine particles (filler).